Many data handling systems, such as those used for communication or for navigation, utilize data based on binary-random-number codes. The receiver portions of such systems generally must perform two functions. First, the receiver must be capable of initially synchronizing a known coded signal generated at the receiver with the transmitted coded signal which is received, i.e., an acquisition function. Secondly, the receiver must continue to keep the two coded signals (i.e., the transmitted code and the locally generated code) aligned, i.e., a tracking function.
Often such functions are performed by effectively separate circuitry each of which handles the acquisition and tracking operations in a generally independent manner. Thus, acquisition is often handled by utilizing a matched filter or a sequential detection scheme, while the tracking function is handled by the use of a feedback loop which, for example, may utilize an early-late detector for developing an error signal.
For the acquisition operation the use of matched filters has an advantage in that the transmitted coded signal can be acquired relatively quickly even with relatively large initial errors between the locally generated code and the received code. The initial error is then reduced to a sufficiently low level in a relatively short time so that an early-late detector loop can then be utilized for the tracking operation. In the presence of high noise levels where the signal-to-noise ratios become relatively low, the ability of the matched filter to provide fast acquisition depends on the sophistication and complexity of the implementation thereof. The use of binary-random-number sequences in such systems aids in the noise rejection operation and relatively low power random-number sequences can be acquired even in the presence of a relatively noisy background. The matched filter hardware is often highly complex and the cost thereof may be prohibitive or at least highly undesirable in many applications.
Sequential detection circuitry which is used for acquisition operations is usually easier to implement but is much slower in its ability to acquire the incoming signal, particularly an incoming signal which is in the presence of relatively high noise and which moves in time. The ability of such a sequential detector to "lock-on" to the incoming signal depends generally on the product of the signal dynamics and the noise which is present. When such product reaches a certain level, the sequential detector normally will lose its capability for providing such lock-on operation.
Further, during the tracking mode, the use of an early-late detector tends to suffer from its inability to track the incoming coded signal once the error exceeds a certain level. In effect, the acquisition range of the known early-late tracking detectors is relatively narrow and, if the noise-dynamics product drives the error outside of such range while the detector is tracking, reacquisition of the data becomes impossible until the noise-dynamics product reduces to a point where the acquisition becomes possible.
In systems where only an open-loop, matched filter is utilized, the broad capture range thereof permits acquisition of the signal but such systems can not provide any tracking capability so that each time the coded signal is transmitted and received the matched filter must reacquire the signal.
In systems where only an early-late, closed-loop detection scheme is utilized, acquisition or re-acquisition takes a relatively long time if the initial error becomes too large since the loop, in effect, must be stepped along to find the signal (i.e., to reduce the initial error to a level at which the loop can lock-on thereto).
An example of a system which utilizes matched filtering techniques for acquiring a random-number coded incoming signal but which acts as an open loop, non-tracking system requiring reacquisition upon the transmission of each coded signal can be found in the text "Introduction to Radar Systems", M. I. Skolnik, McGraw-Hill Book Co., New York, 1962, page 412.
An example of a delay-locked loop system which can act to track a coded signal once it has been acquired is found, for example, in U.S. Pat. No. 4,092,601 (Lee and Cox), issued on May 30, 1978.
An example of a system which operates in two modes, (1) an acquisition mode using matched filter techniques and (2) a tracking mode using a closed loop operation, can be found in the text "Spread Spectrum Systems", R. C. Dixon, John Wiley and Sons, New York, 1976, page 194.
It is desirable to devise a system which will operate not only to acquire the desired incoming coded signal relatively rapidly but also to continue to track such coded signal after "lock-on" is achieved and which may require substantially less complex hardware for implementing the overall operation than is required for open-loop, matched filter operation. Such a system will retain the desired relatively fast acquisition characteristics and avoid the need for reacquisition during the tracking operation. Such system should provide a reliable operation and be capable of acquiring the incoming signal even with relatively large initial errors so as to provide effective operation in many applications at a reasonable cost.